Closed apparatus providing potential fluidization for horizontally conveying powder materials

ABSTRACT

An improved closed apparatus for conveying powder materials from a storage region to at least one region to be supplied. The apparatus includes at least one horizontal conveyor formed by a lower gas flow duct, an upper duct for flow of the powder and gas, and a porous wall separating the ducts. At least one conduit supplies gas to the lower duct, permitting the establishment of a pressure Pf. The conveyor is provided with at least one balancing column the height of filling of which balances the pressure Pf of the potential fluidization gas for the purpose of potentially fluidizing the material filling the conveyor.

This application is a continuation of application Ser. No. 620,311,filed 6/13/84, now abandoned.

The invention concerns a closed apparatus providing potentialfluidisation for the horizontal or inclined conveying of materialshaving the characteristics of fluidisable powders, from a storage regionto at least one region to be supplied therewith, said regions beingspaced from each other.

The invention also concerns a continuous process for transporting andfeeding `reserve containers` with powder substance such as alumina,which makes it possible to supply a packaging processing assembly suchas a bagging apparatus or a container filling apparatus, or a productionassembly such as an extrusion press or igneous electrolysis tanks in aworkshop, from a single region for storing such materials.

The term fluidisable materials is used to cover all the materials, wellknown to the man skilled in the art, which occur in powder form, beingof such granulometry and cohesion that the speed of flow of the airwhich is blow thereinto has the effect, at low speed, of causing theparticles to lose adhesion with each other, and a reduction in theinternal frictional forces. Such materials are for example alumina whichis intended for igneous electrolysis, cements, plasters, quicklime orslaked lime, fly ash, calcium fluoride, rubber filler material, fecula,catalysts, carbon dusts, sodium sulphate, phosphates, polyphosphates,pyrophosphates, plastics materials in powder form, food substances suchas powder milk, flours, etc.

It is well known that such processes have been researched and developedfor fluidised-bed transportation of materials in powder form. Oneexample of many is that which involves feeding alumina to igneouselectrolysis cells for the production of aluminum.

For that purpose, the alumina, being a powder substance which istransported and solubilised in the electrolytic bath, is progressivelyconsumed as the electrolysis operation takes place, and must be replacedas it is consumed, in such a way that the level of concentration ofsolubilised alumina is kept within the suitable limits for operation ofthe cell at full output. Hence, it is necessary to control the amount ofalumina which is introduced into the electrolysis tank in such a waythat operation thereof is not detrimentally affected by an excess or adeficiency of alumina.

Many apparatuses have been proposed by the man skilled in the art anddescribed in the specialist literature, which are intended to permit aregular feed of alumina to the electrolysis tank. A first apparatus, asdescribed in French patent No. 2 099 434, makes it possible to supplyalumina to the electrolysis cell and comprises an alumina storage tankbelow which is disposed a measuring apparatus for regulating the amountof alumina that issues from the tank and finally a means for thefluidised transportation of the alumina, that means also being known bythe term of `air float conveyor`, which is disposed between themeasuring apparatus and the cell to be supplied with alumina, thealumina tank and the measuring apparatus being disposed laterally besidethe cell. The conveyor means is itself provided with outlet pipes forfeeding the cell at a number of points by way of dip tubes which aredisplaced with an `up-and-down` movement, each delivering substantiallythe same amount of alumina.

However, such an apparatus suffers from major disadvantages from thepoint of view of industrial use thereof. In fact, such an apparatus isdesigned in such a way that it can supply only a single electrolysistank whereas there is a need to supply a series of electrolysis tanks,comprising for example 60 units. In addition, that apparatus is providedwith a storage tank that is remote from the single electrolysis tankthat it can feed and hence it gives rise to a substantial response timebetween the moment of a demand being made for powder materials, and themoment at which the electrolysis tank is supplied with such material. Inaddition, that apparatus is provided with a plurality of points at whichthe powder material is introduced into the electrolysis tank, which donot guarantee that the amount of powder material introduced at eachpoint is equal.

In addition to the above-mentioned disadvantages, as the measuringapparatus is disposed upstream of the air float conveyor, the powdermaterial to be conveyed is totally fluidised so that a layer of gasflowing at high speed is generated between the upper surface of thefluidised bed as it moves, and the ceiling member of the air floatconveyor, that layer of high-speed gas entraining therewith the veryfine particles of the powder material being conveyed. In addition, asthe air float conveyor does not have means for removing or dischargingthe gaseous phase, all the fluidisation air is introduced into theelectrolysis tank at the same time as the alumina, thereby increasingthe amounts of very fine particles which are lost by virtue of beingcarried off, at the location of the electrolysis tank.

The problem encountered by the man skilled in the art is the problem oflong-distance transportation of a powder material which is stored in asilo of very large capacity, for the purposes of supplying packaging orprocessing workshops which are at some hundreds of meters from the silo,which problem has been overcome hitherto by using for example movablecontainers, high-pressure pneumatic conveyor means or mechanicalconveyor means.

More recently, another apparatus for supplying alumina to anelectrolysis cell at multiple points was described in U.S. Pat. No.4,016,053. The apparatus which makes it possible to convey a powdermaterial from a storage area to an area in which it is consumedcomprises firstly a primary, fluidised-bed conveyor which is providedwith means for introducing and discharging the gas used for fluidisingand transporting the powder materials coming from the storage area, andto maintain the primary conveyor substantially full of fluidisedmaterials, and then comprises a plurality of coveyors for supplying anddischarging the gas used for fluidising and transporting the powdermaterials coming from the primary conveyor and maintaining the secondaryconveyors continually full with fluidised powder materials, and finallycomprises means for the discontinuous feed of powder materials to eachelectrolysis tank by the force of gravity, each such supply means beingsupplied by way of one of the secondary fluidised-bed conveyors.

Now, and this is a serious disadvantage, the apparatus described in U.S.Pat. No. 4,016,053 is required to keep the powder material in apermanent condition of fluidisation, whether or not there is arequirement for the electrolysis cell to be supplied with material, sothat accordingly, that apparatus consumes substantial amounts offluidisation gas and consequently energy. In addition, and this isanother major disadvantage, in order for the powder materials in theapparatus to remain in a permanent condition of fluidisation, there is,between the upward surface of the fluidised bed and the ceiling memberof the air float conveyor, a flow of gas which moves constantly and athigh speed towards the discharge vents, thereby entraining out of theapparatus up to 10% by weight of the flow of powder material in the airfloat conveyor.

Thus, the arrangements described in the prior art are not capable offully satisfying the user thereof, as they are found to be complicatedand troublesome from the point of view of industrial application, byvirtue of the amounts of energy that they require for consumption, forexample in the case of loading-unloading apparatuses and pneumaticconveyor systems, or by virtue of the losses of powder materials thatthey cause, or by virtue of the use of carriages, handling equipment,travelling cranes, movable feed hoppers, etc.

It is for that reason that the present applicants, continuing theirresearch in this area, developed an apparatus for transporting powdermaterials in dense bed form, which does not suffer from theabove-indicated disadvantages.

The closed apparatus, according to the invention, providing potentialfluidisation, for conveying powder materials from a storage region to atleast one region to be supplied, which comprises, between said tworegions, at least one horizontal or inclined conveyor provided withfluidisation means, and formed by a lower gas circulation duct, an upperduct for circulation of the powder material and the gas, with a porouswall being disposed between the ducts, and at least one pipe forsupplying the lower duct with gas, is characterised in that, for thepurpose of potentially fluidising the powder material filling theconveyor, the conveyor is provided with at least one balancing column,the height of filling of which balances the pressure of the potentialfluidisation gas.

In order properly to understand potential fluidisation which occurs inaccordance with the present invention, it is appropriate to recall whatfluidisation is, as is habitually employed in the prior art forconveying powder materials.

In accordance with the prior art, as may be found for example inabove-mentioned U.S. Pat. No. 4,016,053, the fluidisation gas isintroduced at a given pressure Pf below the porous fluidisation wallwhich separates the low duct for the flow of gas and the upper duct forcirculation of the power material, in a horizontal conveyor. Thefluidisation gas flows through the porous wall then passes between theparticles in the rest condition of the powder material, forming thelayer to be fluidised. The thickness of the layer in the rest conditionis very much less than the height of the upper duct of the conveyor.

As soon as the speed of flow of the gas in the interstitial spaceexisting between the particles is sufficiently high, the particles aremobilised and then lifted, each particle losing its points of permanentcontact with the neighbouring particles. In that way, internal frictionas between particles is reduced and the particles are put into a stateof dynamic suspension. Hence, each particle subjected to the action ofthe gas flow is subjected to a lifting force due to the speed of the gasin contact with that particle, that speed being of the order of the rateof fall of that particle in air.

Accordingly, that results in an increase in the initial volume of thepowder material and at the same time a reduction in apparent density.

According to the invention, the powder material, when there is noinjection of fluidisation gas, completely fills the conveyor apparatus,in particular the upper duct for the flow of powder material in thehorizontal or inclined conveyor. The particles of the powder material,in the rest condition, form a layer of such material to be conveyed, thethickness of that layer being virtually equal to the height of the upperduct. As soon as gas is introduced under the porous fluidisation wallwhich separates the lower duct for the flow of gas from the upper ductfor the flow of powder material, the pressure of the gas being the samepressure Pf as that which would be employed in the prior art forfluidising the powder material, the balancing column is filled with thatsame material to a pressure head which balances the pressure Pf andprevents an increase in the size of the interstitial spaces between theparticles. Accordingly, the provision of the balancing column preventsfluidisation of the powder material present in the horizontal orinclined conveyor, as the applicants have been able to confirm incarrying out their many experiments. In addition, as the interstitialspacing between the particles does not increase, the permeability of themedium in regard to the gas which is introduced at the pressure Pf is ata very low level and limits the gas flow to a very low valve, by virtueof the section of the balancing column. This, and in order to illustratethe phenomenon observed by the applicants, the powder material to beconveyed, which is for example alumina, being subjected to afluidisation pressure Pf of 80 millibars, in the case of the prior artarrangement as can be found for example in U.S. Pat. No. 4,016,053, thegas flow rate corresponding to the pressure Pf, causing fluidisation ofthe powder material, is of the order of 33.10⁻³ m³.m^(-2l) .s⁻¹, whereasin the case of the present invention, and with the same pressure Pf, thegas flow rate is only of the order of 4.10⁻³.m³.m⁻².s⁻¹, which flow rateis too low to be able to cause fluidisation of the alumina throughoutthe conveyor.

The balancing column according to the invention is preferably vertical.It may be mounted on the axis of the conveyor or positioned laterallyand connected to the upper part of the conveyor by and suitable means.The column is generally tubular and its section, in a planeperpendicular to its axis, may be circular, elliptical or polygonal.

By pursuing their research and carrying out their experiments for thepurpose of developing and perfecting the apparatus according to theinvention, the applicants formed and established that the varioustechnical parameters of the balancing column, as well as the manyparameters involved in the powder material to be conveyed, wereassociated in relationships such as to define the minimum total area σthat the cross section or sections of the columns must have, inproportion to the total area of the porous wall: S. Thus, for anapparatus providing potential fluidisation for conveying powdermaterial, in accordance with the invention, comprising a storage region,at least one closed, horizontal or inclined conveyor and at least onecolumn for balancing of said conveyor, with a fluidisation gas pressurePf and a total area S of the porous wall, the minimum total area σ ofthe cross section of the balancing column or columns must comply withthe following relationship:

    σ≧(S/200)

Preferably, the minimum total area σ is selected to lie within thefollowing limits which were determined experimentally:

    (S/200)≦σ≦(S/20)

In the particular case of alumina, the applicants found from experimentthat the minimum total area σ that the cross section of the balancingcolumn or columns must have, must be at least equal to S/100 andpreferably between S/100 and S/50.

The height of the column must be at least equal to the value extractedfrom the equilibrium relationship:

    Pf=H.ρ

in which ρ is the specific weight of the powder material present in thecolumn and Pf is the fluidisation gas pressure.

In general, the apparatus according to the invention is provided withone balancing column but, where the fluidised-bed conveyor isparticularly long, it may be an attractive proposition for a conveyor tobe provided with at least two balancing columns.

The balancing column according to the invention is appliedunrestrictedly to any installation for transporting powder materials,which provides for potential fluidisation, from a storage region to aregion for consumption of such material, comprising a primary,potential-fluidisation conveyor for feeding a plurality of secondary,potential-fluidisation conveyors which are independent of each other,each secondary conveyor feeding a plurality of tertiarypotential-fluidisation conveyors which are independent of each other,each tertiary conveyor feeding the consumption region by way of asuitable storage device. In other words, the balancing column accordingto the invention is successfully applied to an array ofpotential-fluidisation conveyors disposed in a cascade arrangement.

The invention will be better appreciated by reference to thenon-limiting description of FIGS. 1, 2 and 3 illustrating a mode ofoperation and an industrial application of the apparatus.

FIG. 1 is a simplified view in vertical section of the apparatusaccording to the invention, comprising a single horizontal conveyor andshowing the powder material in a rest condition, although the gasenclosed therein is maintained at a potential fluidisation pressure,

FIG. 2 is a simplified view in vertical section of the apparatusaccording to the invention, comprising a single horizontal conveyor andshowing the powder material in a dynamic phase, that is to say,supplying a consumption region which is at a distance from the storageregion, and

FIG. 3 is a perspective view, with some areas partially cut away forviewing, of a complete conveyor installation in accordance with theapparatus of the invention.

Referring to FIGS. 1 and 2, the closed apparatus providing potentialfluidisation for the horizontal or inclined conveying of a powdermaterial comprises an air storage container 1 for the material to beconveyed, which is connected by a conduit 2 to a conveyor 3 of thefluidisation air-float conveyor type, a balancing column 4, a means 9for discharge from the conveyor, and a storage container 10 for theconveyed material, which is to be consumed by the means 11.

The air storage container 1 contains the powder material 12 in loosebulk form, being subject to atmospheric pressure. The container 1operatively communicates with one of the ends of the horizontal (orinclined) conveyor 3 by way of the conduit 2. The conveyor 3 which is ofa straight elongate configuration comprises a porous fluidisation wall 5having a total area S, which separates the lower duct 6 for the flow ofgas and the upper duct 7 for the flow of powder material. Close to theother end of the horizontal conveyor 3 is disposed a balancing column 4,the minimum total area σ of the section of which must be at least equalto S/200 which disposed at said other end, which is remote from thestorage container 1, is a discharge means 9 which converts thehorizontal movement of the powder material into a vertical movement, forfeeding a storage container for storing the conveyed material, which isdisposed below the horizontal conveyor 3. The conveyed material storagecontainer 10 in turn feeds a region (not shown) in which the material isconsumed by the means 11.

FIGS. 1 and 2 will now be compared in order to describe the movement ofthe gaseous fluids and powder materials.

As shown in FIG. 1, the powder material, in loose bulk form, occupiesthe whole of the conveyor apparatus according to the invention, therebyexpressing the fact that the apparent specific weight of the powdermaterial which fills the apparatus in the regions 12, 7, 14, 16, 21, 17and 18 is the same as that of the material when stored in a heap atatmospheric pressure. The upper level of the material in the container 1must always be higher than the upper level reached by the powdermaterial in the column 4 when the apparatus is subjected to a gaspressure Pf. As soon as a gas pressure Pf is applied, by way of theconduit 8, underneath the porous wall 5, which pressure is identical tothat which would be applied in a fluidised-bed air-float conveyor, thepowder material to be conveyed occupies the column 4 up to a level 15,at a pressure head height such as to balance the gas pressure Pf. As thestorage container 10 is sufficiently filled with the conveyed powdermaterial, the outlet 19 of the discharge means 9 is immersed in theregion 20 of the powder material 18. The resulting balanced condition ismaintained as it is as long as the demand for material to be consumed,from the conduit 11, does not cause the outlet 19 to be cleared, byvirtue of a drop in the level of material as indicated at 20.

Referring to FIG. 2, the consumption of powder material stored in thecontainer 10 is such that the level 20 of that material is below theoutlet 19. As soon as the outlet 19 is open, powder material escapes byway of the discharge means 9 and the outlet 19, by virtue ofdecompression of the gas enclosed in the region 22 of the conveyor 3,which is closest to the consumption region downstream of 11 (not shown).As soon as the consumption region has received the required amount ofpowder, the outlet 19 is again closed off, thereby restoring thebalanced conditions shown in FIG. 1. The region 22 is then re-suppliedwith powder by its progressively falling in or collapsing, in anupstream direction, as far as the silo 1.

FIG. 3 shows an installation for supplying alumina to a series ofelectrolytic cells for the production of aluminium. A cell for theigneous electrolysis of alumina requires a regular feed of alumina tothe molten bath as the alumina is consumed, in respect of time, by meansof a storage region disposed above each cell, to feed the bath from aplurality of points.

The closed apparatus according to the invention for conveying aluminafrom a storage to at least one consumption region comprises the meansdescribed hereinafter:

An air storage container 31 for storing alumina, which is of greater orsmaller capacity according to requirements, and which is disposed at aspacing from the electrolysis workshop area, is positioned at a locationon the industrial site which is readily accessible to permit thecontainer to be supplied with alumina by road or rail container or anyother handling means. The container 31 is connected to a primaryconveyor 33 by means of a conduit 32 to provide a gravity feed. Thefirst primary conveyor 33, of potential fluidisation type, the length ofwhich is equivalent to the length of the part of the electrolysisworking area to be supplied, comprises a porous wall 34 having a totalarea S₁, which separates the lower gas flow duct 35 and the upper powdermaterial flow duct 36. The lower gas flow duct 35 is supplied with gasby means of the fan 37 and the conduit 38. The upper duct 36 is providedwith a balancing column 39, the minimum total area σ₁ of the section ofwhich must be at least equal to S₁ /100. The primary conveyor 33 isconnected to secondary potential-fluidisation conveyors 40, by means oflateral tappings 41 and conduits 42. The secondary fluidisationconveyors 40 are of the same type as the primary potential-fluidisationconveyor 33. They are supplied with gas by way of the conduits 43connecting the lower gas flow duct 35 of the primary conveyor 33 to thelower gas flow ducts of the secondary potential-fluidisation conveyors40. Each secondary conveyor 40 may be provided with a balancing column44. Each secondary conveyor 40 extends longitudinally above anelectrolysis tank as diagrammatically illustrated at 45. Lateraltappings 46 supply alumina to dip tubes 47 which discharge the powdermaterial into storage containers 48 disposed above each electrolysistank. The storage containers 48 are possibly provided with means (notshown) for closing off their outlet, such closure means being controlledby the electrolysis cell. Likewise, the ends 49 of the tubes 47 may beprovided with closure means controlled in dependence on the feed to theelectrolysis cell, or may be closed off by an increase in the level ofalumina in the respective container 48.

EXAMPLE (as shown in FIG. 3)

An apparatus for conveying alumina, of potential-fluidisation type, inaccordance with the invention, was constructed, which provided a feed ofthe powder material to 16 igneous electrolysis tanks with a dailyconsumption of 1T/tank/24 hours of alumina over a period of 600 days.The capacity of the storage container 31 was 100 m³ and it contained onaverage 60 tonnes of alumina. It supplied a primarypotential-fluidisation conveyor by a gravity feed at a rate of 0.7 tonneper hour. The primary potential-fluidisation conveyor 33 was of a totallength of 80 meters and was supplied with alumina by means of the aircontainer 31. The height of the conveyor was 0.35 m, and its width was0.12 m, while the area S₁ of the porous wall was 10 m².

The gas flow duct 35 of the primary conveyor was fed by means of the fan37 at a rate of 150 m³ /hour, at a pressure Pf of 0.08 bar.

The primary potential-fluidisation conveyor 33 was provided with fivebalancing columns 39. Each column was 2 meters in height, and the areaof its horizontal section was 0.03 m².

The primary potential-fluidisation conveyor 33 was provided with 16lateral tappings 41, each tapping 41 providing a gravity feed to asecondary potential-fluidisation conveyor 40 at a rate of 1T/tank/24hours of alumina.

Each secondary conveyor 40 was 6 m in length, 0.25 m in height and 0.06m in width. The area of its porous wall was 0.4 m², that wall beingsupplied with gas by the fan 37 at a rate of 6 m³ /hour at a pressure Pfof 0.08 bar.

Each secondary conveyor 40 was provided with two lateral tappingssupplying alumina to the storage containers 48 by way of dip tubes 47.

Each storage container which was disposed above the electrolysis tankthat it supplied had a capacity of 1.2 m³ and contained on average 1 Tof alumina.

When the alumina in each container 48 was in excess, the end 49 of thetube 47 was obviously closed off. In contrast, when the alumina in eachcontainer 48 was below a certain amount, the end 49 of the tube 47 wascleared and the apparatus according to the invention supplied alumina tothe storage container 48.

The conveyor apparatus according to the invention consumed 1.3 KWh for agas flow rate of 250 m³ /hour per tonne of alumina transported, whereas,in accordance with the prior art, the same apparatus but without abalancing column would have required 10.0 KWh for a gas flow rate of2000 m³ /hour per tonne of alumina transported.

We claim:
 1. A method for potentially fluidizing and conveying powder materials in a conveyor located between a storage region and a supply hopper, the method comprising the steps of:(a) completely filling said conveyor with non-fluidized powder material; (b) supplying a gas to the conveyor to permit fluidizing of the powder material; (c) preventing fluidization of the powder material by filling a balancing column in communication with said conveyor with powder material to create a pressure head which counterbalances the fluidization gas pressure; (d) permitting immediate fluidization of the powder upon withdrawal of powder from said supply hopper, due to lowering of the gas pressure in the conveyor,whereby the fluidized powder material is conveyed to said supply hopper until said hopper is refilled, thereby causing said conveyor to return to a counterbalanced condition. 